Laser marking and laser engraving are well-known techniques which are frequently used in preparing identification cards and security documents. However in literature, laser engraving is often incorrectly used for laser marking. In laser marking, a colour change is observed by the local heating of material causing carbonization. Gray shades can be obtained by varying the beam power. In laser engraving, the material is removed by ablation.
It is frequently mentioned in the literature that polycarbonate, PBT and ABS as polymers are laser-markable as such, i.e. in the absence of a so-called “laser additive”. However, laser additives are often added even in the case of these polymers in order to improve the laser markability further. A laser additive is a compound absorbing light at the wavelength of the laser used, usually at 1064 nm (Nd:YAG), and converting it to heat.
Carbon black can be used as a laser additive, however carbon black has a degree of colour which is sufficient to be visible prior to application of the laser beam and that can be unsightly or interfere with the distinctness of the mark after the laser beam has been applied. These disadvantages lead to a search for more efficient “colourless” laser additives. For example, U.S. Pat. No. 6,693,657 (ENGELHARD CORP) discloses a YAG laser marking additive based on a calcined powder of co-precipitated mixed oxides of tin and antimony which will produce a black mark contrasting with the surrounding area when exposed to YAG laser energy but prior thereto does not impart an appreciable colour to the surrounding area or cause a significant change in the performance of the material in which it has been added. Generally, the alternative laser additives are based on heavy metals making them less desirable from an ecological viewpoint.
Today, the most common plastic used in laser marking identification cards and security documents is a foil of extruded polycarbonate. However, polycarbonate foils have a number of disadvantages. The most important ones are their brittleness, leading to security cards getting broken when bended, and their lack of inertness towards organic solvents, opening possibilities to falsify a security card.
Polyethylene terephthalate (PET) exhibits a high solvent resistance, a high flexibility and is less expensive than polycarbonate, but exhibits no or very poor laser markability.
EP 866750 A (SCHREINER ETIKETTEN) discloses laser-markable films for labels based on a white PET film which bears a black coating. Laser irradiation ablates the black coating and uncovers the white background. This structure enables good high-contrast white-on-black inscriptions and drawings.
U.S. Pat. No. 7,541,088 (MITSUBISHI POLYESTER FILM) discloses a biaxially oriented, heat-set, at least two-layer coextruded film formed from polyethylene terephthalate (PET) or polyethylene 2,6-naphthalate (PEN) including a base layer and at least one outer layer. The base layer includes a white pigment and a laser absorber which has been coated with a carbonizing polymer. It is disclosed at col.3, lines 64-66 that only the combination of the laser marking additive with a white pigment and with a specific coextruded layer structure leads to effective laser marking. The opaque coextruded layer structure prevents any security print, such as e.g. guilloches, present on a foil beneath to be visible through the laser markable layer structure.
EP1852270 (TECHNO POLYMER) discloses a laminate for laser marking comprising: a layer A comprising a multi-color developing laser marking thermoplastic polymer composition capable of producing markings having two or more different color tones by irradiating thereto two or more laser lights having different energies from each other, the composition satisfying the following requirements (1) and (2): (1) comprising a chromatic colorant, a black substance capable of being dissipated by itself or discolored when exposed to the laser lights, and a thermoplastic polymer at the following mixing ratio, and (2) containing the chromatic colorant and the black substance in amounts of from 0.001 to 3 parts by weight and from 0.01 to 2 parts by weight, respectively, on the basis of 100 parts by weight of the thermoplastic polymer; and a layer B formed on at least one surface of the layer A, the layer B comprising a transparent thermoplastic resin, and exhibiting a light transmittance of not less than 70% as a single layer.
EP792756 (NIPPON KAYAKU) discloses a laser marking article having a cured film of a liquid composition comprising an energy beam-curable resin, a leuco dye and a developer and has a viscosity of not more than 20 cps as measured by an E type viscometer at 60° C.
Laser markable security films prepared by coating a laser markable layer on a support offer a number of advantages. In manufacturing these films, changes to the laser markable layer, e.g. in composition and thickness, or addition of other layers, e.g. a specific adhesion layer, can be implemented much easier than by an extrusion process. However, a number of physical properties, which are guaranteed by an (co)extrusion process, are not self evident for coated layers. There is a need for laser markable security films prepared by coating, which exhibit good physical properties for curl, adhesion and cracks on bending.